Due to the special relevance of the invention for pressure measurement cells, the invention will be explained based on pressure measurement cells as an example of its application.
Pressure measurement cells according to the state of the art combine a ceramic measuring membrane and a ceramic platform, wherein the measuring membrane is connected pressure-tightly with the platform along a peripheral joint, which contains an active hard solder, or braze, wherein a pressure chamber is formed between the measuring membrane and the platform, wherein the equilibrium position of the measuring membrane results from the difference between a pressure reigning in the pressure chamber and a pressure acting on the outside surface of the measuring membrane, thus its surface facing away from the pressure chamber.
Serving as material for the platform and the measuring membrane are especially aluminum oxide ceramics, which, due to their elastic properties and their media resistance, are suited for the manufacture of pressure measurement cells. The mentioned ceramic components are especially joined with an active hard solder, or braze, which is preferably an active hard solder, or braze, containing Zr, Ni and Ti. The manufacture of such an active hard solder, or braze, is disclosed, for example, in European Offenlegungsschrift (laid open application) EP 0 490 807 A2. According to the method described there, especially rings of the active braze, material can be manufactured, which are positioned between measuring membrane and platform, in order to solder, or braze, these with one another.
For joining the components, for example, the ceramic bodies are heated with an intermediately lying, solder preform in high vacuum to a temperature, which melts the active hard solder, or braze, so that a reaction begins between the active hard solder, or braze, and the ceramic bodies. By cooling, the active hard solder, or braze, solidifies and the reaction between the active hard solder, or braze, and the ceramic body is stopped. During cooling, however, different coefficients of thermal expansion of the material of the ceramic body, on the one hand, and of the active hard solder, or braze, on the other hand, can lead to considerable stresses between the ceramic components and the joint, since after solidification of the solder there is still a cooling of several hundred K.
In selecting an active hard solder, or braze with a coefficient of thermal expansion suitable for the ceramic material, however, attention must be paid that the suitable active hard solder, or braze, does not have too high a melting point and, thus, not be suitable for joining the ceramic components, for example, because electrodes, which are provided on the ceramic bodies cannot withstand such temperatures.